I want to share my favourite method for removing heavy rust. I think I have tried all the common techniques even using chelating agents.
My favourite is electrolysis and I have tried many different witches brews, here is my favourite.
It is simple, get a 5 or 6 gallon bucket and fill 2/3 full with water. Added 10 lbs sodium carbonate ( aka washing soda, soda ash) ****avoid using baking soda or sodium bicarbonate they are NOT the same*****. Inexpensive sodium carbonate can be purchased at a pool supply store. it is used as a buffer to lower pH. I believe I paid approx $8-10 for 10lbs. I have use mine for over a year now without replacing the soda or electrode.
Add all the soda ash to your water, you want it saturated and you want some white powder sitting on the bottom. Grab a clean piece of steel as your positive electrode and place at one side of the bucket. Connect the red positive clamp from a battery charger to the this steel electrode. Place the charger away from the bucket in a safe place where it can't fall into the bucket. Be safe, you are using electricity.
We are now set up. I recommend to remove any grease from the item to be cleaned. I use mineral spirits and allow to dry. If big chunks of rust can be removed easily, do it now.
Grab a heavy stiff wire like a piece of coat hanger and dangle the part in the solution on the opposite side of the bucket than the red electrode. Connect the black negative charger clip to the stiff wire for the part to be cleaned. Make sure your parts DON'T touch the steel electrode (red/positive). Plug in the charger and I use 6V which draws 5 Amps with my setup and I leave overnight. I find 12V creates too much foam. Small parts can be placed in a small deep fryer basket, for example
the good news is the Fe+3 ions are reduced by the electrons from the negative power supply/charger to produce iron metal as finely divided black powder at the metal/rust interface. After 6-10 hours or so I unplug the charger and clean the parts up with a stiff brush and usually all the rust just falls off. At times, I may need to repeat on some parts. I usually dry the parts after washing to reduce quick flash rusting of the fine iron particles. A quick coat of oil and we are good to go.
BTW, this method does not corrode your parts like some other methods.
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